1. Field of the Present Invention
The present invention relates to methods and apparatus for measuring a property of at target, such as overlay, usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
The targets used by conventional scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). This simplifies mathematical reconstruction of the target as it can be regarded as infinite. However, in order to reduce the size of the targets, e.g., to 10 μm by 10 μm or less, e.g., so they can be positioned in amongst product features, rather than in the scribe lane, metrology has been proposed in which the grating is made smaller than the measurement spot (i.e., the grating is overfilled). Typically such targets are measured using dark field scatterometry in which the zeroth order of diffraction (corresponding to a specular reflection) is blocked, and only higher orders processed. Examples of dark field metrology can be found in international patent applications WO 2009/078708 and WO 2009/106279 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in patent publications WO 2011/012624 and WO 2011/023517 and WO 2012/062501. The contents of all these applications are also incorporated herein by reference in their entireties. Diffraction-based overlay (DBO) using dark-field detection of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple targets can be measured in one image.
In the known dark-field metrology technique, overlay measurement results are obtained by measuring the target twice under certain conditions, while either rotating the target or changing the illumination mode or imaging mode to obtain separately the −1st and the +1st diffraction order intensities.
In the special case when an overfilled target is located in an environment that is empty of features, such as a non-reflective background, then, instead of dark-field metrology, angularly-resolved metrology can be used to measure the target to obtain separately the −1st and the +1st diffraction order intensities at the same time in the pupil plane of the inspection apparatus. In this case the intensity measured in the pupil plane is not contaminated by light scattered from the environment around the target.
It is known that in scatterometers, wafer stage positioning error is such that a target can incur a random position error (for example +/−3 μm) with respect to the measurement spot center in each wafer stage movement. This means that the target grating will not always be centered with respect to the measurement spot. The measured overlay has been found to vary from the value measured with the target at the center of the measurement spot. This overlay measurement error has been found to be linear (at least in part) and anti-symmetric with respect to displacement from the center of the illumination spot, as discussed below in relation to FIGS. 7 and 11. Therefore, when performing overlay measurements, in order to minimize overlay measurement error the target grating should be centered with respect to the illumination spot. If the target is not centered, then the overlay measurement will have systematic errors introduced. For composite targets with multiple grating orientations and/or overlay biases, it is not possible to have all the gratings centered in the illumination spot simultaneously.